Modulator



Patented June 28, 1949 MODULATOR,

Frank A. Leibe, Quakertown, Pa., and Benjamin B. Mahler, Newark, N. Jasslgnors to- Federal Telephone and Radio Corporation, New York, N. Y.,a corporation of Delaware Application May 10, 1947, Serial No. 747,21412 Claims. (01. 332-29) Our invention relates to modulators and is.particularly directed to means for frequency modulating oscillators ofthe phase-shift type.

In teletype transmission, via wire lines or radio, the telegraphicsignals are preferably applied, by keying, to a low frequency source,the marking and spacing impulses being characterized by differentfrequencies which are easily distinguished by filters at the receivingend. The marking and spacing frequencies, are preferably inthe audiorange and for best results should be constant in amplitude and free oftransients or key-clicks.

The desired low frequencies can not be easily generated with electronicoscillators having the usual inductance-capacitance tuning circuits. Asthe lower reaches of the audio frequency spectrum are entered, thetuningcoils and condensers become prohibitively large and expensive. Beatfrequency generators are complex and not particularly stable as toamplitude or frequency.

The object of our invention is an improved low frequency generator andmeans for modulating the frequency of the generator with telegraphicsignals, the modulated output being constant in amplitude and free oftransient currents.

Another object of our invention is a low frequency oscillator-modulatorthat is stable in frequency and amplitude and is relatively inexpensive.

Our invention is defined in the appended claims and one embodiment isdescribed in the following specification and shown in the accompanyingdrawing, in which the single figure of the drawing is a wiring diagramof an oscillator and modulator according to our invention.

The particular oscillator to which our inven tion is applied in thisdescription comprises an amplifier tube l with anode 2, cathode 3 andcontrol grid 4. A tube with screen and suppressor grids may be used ifdesired. The amplifier I is resistance coupled to amplifier tube 5.

For self sustained; oscillations in amplifier tube I, the output circuitis coupled to the input circuit through a capacity-resistance network 6with the required number of meshes to shift the anode voltage applied tothe grid 180 electrical degrees. The network shown comprises fourmeshes, each with a shunt resistor I and series capacitor 8. Oneresistor is tapped to divide the resistor into two portions, Ia and 1b.The operating frequency of such an oscillator is a function of theresistances R and capacities C of the network and may be written F=K/RC,where K is a constant determined by the circuit parameters. decrease inthe ohmic value of one or more re- 2 sistors of the phase-shift networkwill cause an increase in frequency, and an increase in resistance willlower the frequency.

We propose to change the effective value of one of the resistances ofthe network, such as lb, at keying rate to change the frequency ofoscillations between two values for marking and spacing signals. Wepropose to connect the anodecathode space of an electron dischargedevice 9 across resistor lb so that the plate resistance of said devicemay shunt the resistance of resistor lb. By alternately making theelectrondischarge device conducting and nonconducting, the effectiveresistance between the ends of lb is made, I respectively, substantiallyzero or the full value of vice and a single-pole double-throw switch His connected in circuit with the source to apply either a high positiveor a high negative voltage to the cathode. magnetically controlled by amanual key or a teletype sender. A condenser 12 is connected be tweenthe cathode and ground so that when the discharge device is madeconductive, by applying a high negative voltage to the cathode,resistor, lb iseffectively short circuited for alternating voltages. Bychanging the cathode voltage from negative to positive all direct andalternating components of space current are interrupted and resistor 1bis restored to the oscillator circuit. It will thus be seen that the twopositions of the switch produce two values of resistance in the RCnetwork and two frequencies of oscillation. Either frequency may becalled the marking frequency and the other the spacing frequency.

According to a further and important feature of our invention, thetransition between marking and space frequencies is made withoutdisturbing transient currents. A resistor l3 in series with the keyingvoltage and the condenser l2 comprise an RC time-constant circuit thatwill delay changes in potential of the cathode and will insure gradualchanges in frequency. So that the charging and discharging pattern ofcondenser I! may be the same, to make marking and spacing signalssymmetrical in shape, another electron discharge device M is placedacross the condenser l2. The polarity of the two space paths The switcharmature may be as it does on negative battery thus assuring symmetricalmodulating pulses and eliminating bias, and distortion, by the modulatorcircuit.

Adjustment of the amount of frequency shift is made possible byadjustments of resistors 1a and lb.

set, the polarity of said diode being reversed with respect to saidjunction.

4. In an oscillator circuit comprising a frequency determining impedanceelement, an arrangement for angle modulating said oscillator circuitwith modulation signals from a source comprising an electron dischargedevice circuit, said electron discharge device circuit comprising adiode in series with a condenser, means for coupling said impedanceelement and said electron discharge device circuit in parallel, meansfor applying modulation signals across said condenser for controllingthe conductance of said diode in accordance with the applied signals.

5. An arrangement according to claim 4, wherein said impedance elementis a resistor and said oscillator circuit comprises a.resistance-capaci- In one oscillator we constructed, the amplifier tubei was a commercial pentode known as a GSJ'l, and the phase-shift networkwascomposed of 200,000 ohm resistors and .0004 microfarad condensers.The mean frequency of oscillations was about 2375 cycles per second, andwas made to shift about 100 cycles above and below this value formarking and spacing frequencies, respectively. Discharge devices 9 andII were combined in one envelope, the double-diode known as the "6H6"being found well suited for the purpose. 130 volts positive and 130volts negative at the terminals of direct current source l0 were foundconvenient to switch the diodes in normal keying, a conventional powerrectifier and filter being used for this source.

Our improved oscillator-modmator is relatively inexpensive tomanufacture. The frequency determining circuit of the oscillator iscomposed of resistors and condensers which, in the low frequency rangementioned, are small andv cost but a small fraction of the cost of theelements of any of the conventional inductance-capacity tuning circuits.Yet the frequency and amplitude stability of our oscillator isremarkably good. The temperature coefilcient of resistance of resistorscan be negligible, and fixed condensers can be sealed to be practicallyindependent of temperature and humidity changes. Our novel modulator,with its time delay features, goesfor smooth keying and in combinationwith the oscillator disclosed may be operated at high speeds withoutundesirable transients or distortion.

We claim:

l. A signalling system comprising an oscillator having a frequencydetermining circuit, a resistor in said circuit, a diode and a condenserconnected in series across said resistor, a source of positive andnegative voltages connected to the junction of said diode and condenser,and a switch operable at signal frequency selectively applying saidpositive and negative voltages to said junction.

2. The signaling system defined in claim 1 further comprising aresistance element connected in series between said junction and saidsource of voltage. l

3. The signaling system defined in claim 1 further comprising aresistance element connect- -"ed in series between said junction andsaid source to voltage, a second diode and a second resistance elementconnected in series across said condentance across said regenerativecircuit including said resistor.

6. An arrangement according to claim 4, wherein said impedance elementcomprises a resistor.

7. An arrangement according to claim 6, com

prising means for coupling one end of said resistance to the plateelectrode of said device. means for coupling the cathode of said devicethrough the condenser to the other end of said resistor. 8. Anarrangement according to claim 7, further comprising a second electrondischarge device circuit coupled between the junctions of said firstdevice and the condenser, and the other end of said resistor.

9. -An arrangement according to claim 8, wherein said second dischargedevice circuit comprises a diode, means for coupling the plate electrodeof said second device to said junction point and its cathode electrodethrough a resistance to the other end of said resistor.

10. An arrangement according to claim 9, wherein said source comprises asource of positive and negative potentials, means for applying saidpositive and negative potentials across said condenser in accordancewith the intelligence to be transmitted.

11. An arrangement according to claim 10, wherein said last namedapparatus comprises a resistor and switching means for applying saidpositive and negative potential through said last named resistor acrosssaid condenser.

12. An arrangement according to claim 11, wherein the value of theconductance of the diode and resistance of said second discharge devicecircuit is adjusted to place a charge on the condenser havingsubstantially 'the same but of opposite polarity as the charge placed onthe condenser by said first discharge device circuit.

FRANK A. LEIBE. BENJAMIN B. MAHLER.

REFERENCES CITED The following references are of record in the die ofthis patent:

UNITED STATES PATENTS

